In the humidification process, steam is normally discharged from a steam source as a dry gas. As steam mixes with cooler duct air, some condensation takes place in the form of water particles. Within a certain distance, the water particles are absorbed by the air stream within the duct. The distance wherein water particles are completely absorbed by the air stream is called absorption distance. Another term that may be used is a non-wetting distance. This is the distance wherein water particles or droplets no longer form on duct equipment (except high efficiency air filters, e.g.). Past the non-wetting distance, visible wisps of steam (water droplets) may still be visible, for example, saturating high efficiency air filters. However, other structures will not become wet past this distance. Absorption distance is typically longer than the non-wetting distance and occurs when visible wisps have all disappeared and the water vapor passes through high efficiency filters without wetting them. Before the water particles are absorbed into the air within the non-wetting distance and ultimately the absorption distance, the water particles collecting on duct equipment may adversely affect the life of such equipment. Thus, a short non-wetting or absorption distance is desirable.
Steam dispersion systems that utilize a single tube configuration normally have long non-wetting or absorption distances. Steam dispersion systems that utilize designs with a plurality of closely spaced tubes with hundreds of nozzles achieve a short non-wetting or absorption distance. However, such designs may create significant amounts of unwanted condensate. Depending upon the type of steam dispersion system, there have been a number of different methods utilized in the prior art for disposing of unwanted condensate.
In discussing condensate removal, there are two basic types of steam dispersion humidifying systems, one that uses non-jacketed dispersion tubes, herein referred to as a “Steam Dispersion Tube Panel” system, and another that uses a steam jacket wrapped around each dispersion tube, herein referred to as a “Steam Injection” system. Virtually, in all systems, some steam condenses into liquid water as it flows within the humidification system prior to being dispersed into the space requiring humidification. Steam Dispersion Tube Panel systems can be used with either atmospheric pressure steam or pressurized boiler steam. The condensate that forms within a Steam Dispersion Tube Panel system is collected in a manifold (e.g., a header) and may be drained to a P-trap where it is either discharged to a drain via gravity, returned to an atmospheric steam generator via gravity, or collected and pumped back to the atmospheric steam generator or boiler condensate collection point with condensate pumps.
Steam Injection type humidifiers are used with boilers since they employ a steam jacket within which flows boiler steam, normally at about 5 psi to 60 psi. The steam jacket wraps around each dispersion tube and vaporizes condensate forming within the dispersion tube, thus, eliminating the need to drain condensate at atmospheric pressure out of the dispersion tubes. The energy to vaporize the condensate within the dispersion tubes comes from condensing an equivalent mass of steam within the steam jacket. Since the steam jacket is under pressure, the condensate within the steam jacket is returned to the boiler without the restrictions, costs, and the piping complexity imposed by P-traps, proper slopes for draining, installation/maintenance of condensate pumps, and possible confusion involved with various steam piping, some of which may be operating at atmospheric pressure and some of which may be operating at boiler pressure. Some examples of Steam Injection type systems can be found in U.S. Pat. Nos. 3,386,659; 3,642,201; 3,724,180; 3,857,514; 3,923,483; 5,543,090; 5,942,163; 6,227,526; 6,485,537; and Des. 269,808.
Steam Dispersion Tube Panel systems have less heat gain to the duct air, and, thus, waste less energy, compared to Steam Injection systems, since there are no steam jackets exposed to the air flow. The surface temperatures are also lower than the surface temperatures of the steam jackets. They also have shorter absorption distances since the absence of steam jackets allows the dispersion tubes to be more closely spaced. Given comparable capacities and absorption distances, a Steam Dispersion Tube Panel system will also have less static air pressure drop across the assembly than a Steam Injection system. However, the condensate from Steam Dispersion Tube Panel systems is often wasted to a drain due to the cost and maintenance of using condensate pumps. Additionally, the clearance needed below the bottom of a Steam Dispersion Tube Panel system for a P-trap is often difficult to accommodate, as is the piping exiting the P-trap, which is normally sloped.
Steam Injection systems seldom waste condensate to a drain as the condensate is pressurized and returned to the boiler without the cost and maintenance problems of condensate pumps or the clearance problems of P-traps and sloped drain lines. However, Steam Injection systems have more heat gain, and, thus, waste more energy than Steam Dispersion Tube Panel systems. They also have longer absorption distances and more static air pressure drop than comparable Steam Dispersion Tube Panel systems.
It is desirable for a humidification system that possesses the advantages of both a Steam Dispersion Tube Panel system and a Steam Injection system without any of their associated disadvantages.